This invention relates in general to recirculating document feeders for use with electrostatographic reproduction apparatus, and more particularly to a recirculating document feeder having improved document sheet handling reliability by use of sequential control of the document sheet transport mechanisms of the feeder.
In order to increase the productivity and ease of use of electrostatographic reproduction apparatus, it has been common practice to provide such apparatus with automatic document set handlers. Early automatic document set handlers accepted a document set stack and removed individual document sheets from the stack one at a time (see U.S. Pat. No. 3,747,918, issued Jul. 24, 1973, in the name of Margulis et al). The removed document sheet was delivered to an exposure station of the reproduction apparatus where the desired number of reproductions of such document sheet were made. Thereafter, the document sheet was returned to the stack and the next document sheet was delivered to the exposure station. Such sequence of document sheet feeding and reproduction necessitated the use of an auxiliary sorter device in conjunction with the reproduction apparatus to provide collated reproduction sets corresponding to the document set. The use of a sorter device added to both the complexity and expense of the reproduction operation.
More recently, automatic document handlers typically referred to as recirculating document feeders have been developed. Recirculating document feeders, such as shown for example in U.S. Pat. No. 4,169,674 (issued Oct. 2, 1979, in the name of Russel) deliver document sheets seriatim to the reproduction apparatus exposure station and return the sheets to the document stack in order. At the exposure station, only one reproduction of each respective document sheet is made on one circulation. The desired number of reproductions is made by recirculating the document sheets from the stack to the exposure station and then back to the stack a corresponding number of times. By such reproduction sequence, the reproduction set of the document set is received at an output hopper in collated order. Thus no subsequent operational steps on the reproduction set are required.
While recirculating document feeders have proven very popular in that they enhance productivity and increase ease of use of the reproduction apparatus, they require complex construction to reliably recirculate the document sheets and effectively handle the document sheets in a manner to prevent damage thereto. Additionally, with the rapid transport of document sheets in high productivity recirculating document feeders, precise control of sheet transport to assure proper timing of the necessary sequence of events to make desired reproductions is difficult to accomplish. Different document sheet types exhibit different transport characteristics, and the transport mechanisms themselves change operating characteristics over time due to wear. Accordingly, previous known high productivity recirculating document feeders required repeated intricate adjustment.